1. Field of the Invention
This invention relates to a process suitable for forming a functional deposited film containing silicon, particularly a deposited film containing polycrystalline silicon to be used for semiconductor device, photosensitive device for electrophotography, line sensor for image input, image pickup device, etc.
2. Related Background Art
For example, for formation of an amorphous silicon film, the vacuum deposition method, the plasma CVD method, the CVD method, the reactive sputtering method, the ion plating method, the optical CVD method etc., have been attempted to be practiced, and, in general, the plasma CVD method has widely been used and industrialized.
However, for the deposited film constituted of amorphous silicon, there is room left for further improvement of overall characteristics with respect to electrical, optical characteristics and, fatigue characteristic or use environmental characteristic in repeated uses, further productivity and bulk productivity including uniformity and reproducibility.
Particularly, depending on the application use of the deposited film, bulk production with reproducibility must be attempted with full satisfaction of enlargement of area, uniformity of film thickness as well as uniformity of film quality and yet at high speed film formation, and therefore in formation of an amorphous silicon deposited film according to the plasma CVD method, enormous installation investment is required for bulk production equipment and also control items for such bulk production become complicated, with control tolerance being narrow and conditioning of the equipment being delicate. These are pointed out as the problems to be improved in the future.
On the other hand, in formation of a polycrystalline deposited film, the normal pressure CVD method or the LPCVD method have been employed, but these methods require high temperature and therefore substrate materials are limited to those having high melting points. On the other hand, due to difficulties in control of crystal grain size, compensating for the defects existing at the grain boundaries or orienting regularly the crystal faces, there have been pointed out problems such that no practically useful characteristic can be obtained, that enlargement of area is difficult, that no uniform film thickness or film quality can be obtained and that reproducibility is poor.
Also, in the plasma CVD method, although substrate temperature can be made lower as compared with the normal pressure CVD method or the LPCVD method, on the other hand, the structure of the reaction device is limited for formation of stable plasma, and besides a large number of other parameters (gas species introduced, gas flow rates, pressure, high frequency power, evacuation speed, etc.) also exist, which have respectively great influences on plasma. Accordingly, plasma will be delicately varied by slight changes in these parameters or their combinations, whereby bad influences were frequently exerted on uniformly or electrical and optical characteristics of the deposited film. Further, the damage by collision of electrons or ions existing in plasma against the deposited film is also great, which is one of the factors deteriorating film characteristics.
As described above, under the present situation, in both of amorphous silicon films and polycrystalline silicon films, problems attributable to their forming methods exist. Particularly, in aspects of improvement of electrical characteristic of deposited film as well as stability, it has been desired in formation of a polycrystalline silicon film to develop a method for forming a polycrystalline silicon film with large area at low cost while maintaining its practically useful characteristics, uniformity and stability.